By the late 1930s
and early 1940s Simpson, though relatively young, was already a
distinguished paleontologist at the American Museum of Natural
History in New York City. His achievements included a Yale doctorate
in geology and paleontology, position as a visiting research scholar
at the British Museum, leader of two year-long fossil-collecting
expeditions to Patagonia, author of two books and more than one
hundred scientific articles and monographs, and newly elected fellow
of two of the most distinguished honorary and scholarly societies,
the American Philosophical Society and the National Academy of
Sciences. As new discoveries, ideas, and theories in genetics were
being published, Simpson understood their importance and kept himself
informed. Before leaving for military service in late 1942, Simpson
completed a major revolutionary text entitled Tempo and Mode in
Evolution, published
two years later.

Simpson's book applied the
concepts and conclusions of the new discoveries in genetics to the
large body of fossil evidence of life's long history, and claimed
that the "microevolution" of the geneticist could indeed be
extrapolated to explain adequately the "macroevolution" of the
paleontologist. That is, the mechanisms of generating and
accumulating inherited variation, as described by laboratory
geneticists and field naturalists, were sufficient to provide a
parsimonious explanation of the adaptations, specializations, and
evolutionary trends of the paleontologists, as measured in their
fossils over long intervals of geologic time. In this respect
Tempo and
Mode became one of a
half-dozen books that formed the basis for what came to be called the
modern evolutionary synthesis. "Synthesis" because the new body of
theory came from a variety of fields--genetics, ecology, anatomy,
field biology, paleontology, botany as well as zoology, and
biogeography-- which were integrated into a unified whole.

Simpson thereby
single-handedly brought the discipline of paleontology into the
mainstream of biological research by validating fossil evidence for
solving evolutionary questions. Before Simpson, what fossils had to
say to biologists as articulated by paleontologists was at best
confusing, at worst contradictory, when compared to their own
observations and theories. Simpson debunked, once and for all, many
previous paleontological explanations of evolutionary phenomena that
depended upon inherent or internally directed forces, like "momentum
and inertia," which argued that once organisms began to evolve they
continued to do so because of the momentum of past evolution; "racial
senescence," the notion that organisms in a given line may exhaust
their evolutionary reserves and become extinct; "orthogenesis," that
organisms evolve toward some future goal and thus intermediate stages
exist only as steps toward that goal rather than as viable ends in
themselves; and "aristogenesis," that organisms are driven forward in
their striving for perfection. Simpson demonstrated that such
explanations were not consistent with modern genetic theory. He also
provided the coup de grace to lingering claims for the inheritance of
acquired characteristics through use and disuse. Thereafter, if
paleontologists were to carry conviction, they had to ground their
interpretations of macroevolution in terms of microevolution.

Another equally important
contribution of Tempo
and Mode was
Simpson's identification of significantly varying rates of
evolution--very fast, average, and very slow--and his explanation of
how such differing rates would yield characteristic patterns of
evolution within the fossil record (hence the title "Tempo and Mode
in Evolution.") Simpson invoked a special importance for the
environment, with all its physical, chemical, and biologic
manifestations, in influencing evolutionary patterns and rate.

Simpson was thus preeminent
among paleontologists and evolutionary biologists for the two decades
following World War II; indeed he was a household word for these
scientists. And as often happens when fame in a field is so
great--even a field as arcane as paleontology--it spills over into
public consciousness. Thus, in the 1950s and 1960s, Simpson appeared
on the cover of the Saturday Review of Literature, was the subject of a full-page
cartoon in the New
Yorker, was featured
in a radio broadcast by Lowell Thomas, and was periodically mentioned
in the national newspapers, especially his home town papers,
The New York Times,
Herald-Tribune, Sun,
and World-Telegram. He even appeared on early television, guiding an
anchorman through the fossil displays at the American Museum.

Despite this public exposure
and despite his many writings--both professional and popular--George
Gaylord Simpson was a difficult man to know. To most people, even
those colleagues with whom he worked closely, he seemed reserved,
often aloof, extremely guarded about his private life, and capable of
sharp critical comment. He did not make friends easily, and those
whom he referred to as good friends in his autobiography were
surprised to be so considered. Because Simpson made no special
efforts to cultivate friendship among his many acquaintances, he put
most people off. Clearly more brilliant and more renowned than most
around him, he accentuated the distance between them by his lack of
warmth. By the time he died, some of his colleagues of his own
generation had long given up knowing him; others, through hearsay,
wrote him off as cranky, difficult, even embittered.

Toward the end of his life,
Simpson's reputation waned somewhat, in part because his
contributions became so thoroughly assimilated into current theory
and practice that the identity of the originator was forgotten.
Moreover, the next generation of evolutionists, especially among
paleontologists, began to question some of Simpson's conclusions. In
particular, some current paleontologists challenge the idea that
macroevolutionary events portrayed by fossils are merely long-term
extrapolation of the short-term microevolutionary processes seen by
the experimentalists and naturalists. Whatever history's final
evaluation of Simpson's contribution may be, during much of his
lifetime he was judged by many of his peers to be the leading
paleontologist and one of the key founders of modern evolutionary
theory.